For many years now, ion implantation has been used to great effect for the controlled doping of semiconductors. The material to be implanted is first ionized, an ion current is extracted from the ion source and the ions are accelerated by the application of high voltage to form a beam that strikes the target (normally a semiconductor wafer) wherever intended. Depending on their arrival energy, the ions will penetrate some distance below the surface of the target before coming to rest, usually in interstitial positions within the semiconductor lattice where they are inactive. This is normally followed by a rapid thermal anneal (RTA) step during which the implanted donor and/or acceptor ions migrate to nearby substitutional locations in the lattice and become active there.
Referring now to FIG. 1 we show there a schematic plot of concentration (of implanted species) as a function of depth (below the semiconductor surface). Curve 11 represents the form obtained from implanting a single ion species all of whom arrive at the surface with the same energy. Exactly how far any given ion penetrates will vary, depending on exactly where the incoming ion enters the lattice. For example, an ion that collides head-on with a lattice atom may be reflected back from the surface or it may be deflected sideways and follow a path that is more nearly parallel to the semiconductor surface than normal to it. Alternatively, an ion arriving between two lattice atoms may, in some cases, find itself in an intra-lattice channel and continue for a substantial distance before coming to equilibrium.
Thus, as seen in curve 11, the concentration profile that results from conventional ion implantation has the shape of a distribution curve that peaks at a depth corresponding to the magnitude of the acceleration voltage that was applied to the beam. For the purpose of optimizing the electrical performance of devices the idealized concentration profile (obtainable by simulation) is, in general, different from the actual implantation distribution curve. For example, the ideal curve for a particular device might be as outlined by dashed line 12, i.e. flat out to the desired depth and then falling to zero beyond it. A straignt profile of this sort cannot be obtained from one implantation.
FIG. 2 illustrates the individual implantation profiles that are typically achieved in the formation of an NPN bipolar transistor. Curve 21 is flat as it represents the profile of the bulk N-type semiconductor material itself. A layer of P-type material was then implanted (to form the base region) and had a profile corresponding to curve 22. Finally, the emitter region was formed and had a profile corresponding to curve 23.
At any given depth, the contributions to the conductivity from all the various implants at that depth add up, with N and P dopants cancelling one another. Thus, the final concentration profile has the appearance shown in FIG. 3, with net conductivity due to either species gradually reaching zero as the actual P-N junctions (between 31 and 32 or 32 and 33) are approached. In an ideal device the concentrations would remain constant until the junction was reached and then would immmediatedly change to the opposite conductivity type. It would therefore be a distinct advance in the device fabrication art if concentration profiles could be tailored into any desired form, particularly flat profiles.
After searching the prior art, we were unable to find any references that teach the formation of custom profiles through ion implantation. References that we found to be of interest include Glavish (U.S. Pat. No. 5,481,116 January 1996 and U.S. Pat. No. 5,672,879 September 1997) who discloses systems and methods for generating time varying magnetic fields. These systems are used, inter alia, for magnetically scanning an ion beam and an ion implantation system.
Weisenberger (U.S. Pat. No. 4,831,270 May 1989) describes an ion implantation system that is well suited to treating a large number of wafers at a time without being subject to charge buildup problems.